Printed circuit boards (PCBs) are typically made of multiple layers of insulated and conductive materials. Electronic components are mounted to the PCB to form a printed circuit board assembly (PCBA). To mount an electronic component to the PCB, an electronic component includes components leads, or pins, which are inserted into through-holes in the PCB. The pins are then mechanically and electrically connected to the PCB, such as by a filling the through holes with solder.
A conventional method of filling the through holes is a wave soldering process. FIG. 1 illustrates a conventional wave soldering process. The wave soldering process is applied to a PCB 12 that includes an electronic component 12 with pins 14. The electronic component 12 is mounted on a top surface of the PCB such that the pins 14 are positioned in through holes (not shown) in the PCB 10. At the step 2, flux is applied to a bottom surface of the PCB 10. At the step 4, heat is applied to the bottom surface of the PCB 10 to activate the flux and to prevent or minimize thermal shock when solder is subsequently applied. A convection heater is positioned underneath the PCB 10. This is known as a preheating step. At the step 6, solder is applied. Molten solder is contained in a tank called a solder pot. The molten solder is agitated to have a pattern of waves on its surface. When the PCB 10 is moved over this tank, the solder waves contact the bottom surface of the PCB, and stick to the solder pads and pins via surface tension.
Barrel fill is a measure of how much the through hole is filled with solder. The ideal goal is 100% barrel fill, where the entire through hole is filled with solder. In practice however, the percentage of barrel fill is limited by the solder fluidity as the solder begins to cool immediately upon contact with the bottom surface of the PCB. Additionally, barrel fill issues are becoming more problematic due to new PCB designs that have more and more layers leading to thicker and thicker PCBs. FIGS. 2A-2C illustrate an example of the impact on barrel fill with increasing PCB thickness. FIG. 2A shows the PCB 10 having a first thickness. In this example, the flow of solder 16 into the through hole 18 reaches the dashed line, which corresponds to a barrel fill of approximately 75%. If the PCB thickness is increased, as in FIG. 2B, under the same conditions the percentage of barrel fill drops since the solder 16 flows into the through hole 18′ only to the dashed line. With an even increased PCB thickness, as in FIG. 2C, the percentage of barrel fill drops even further.
Insufficient barrel fill leads to reliability problems. Solder in the through hole provides mechanical support for securing the electronic component to the PCB, as well as provides electrical connectivity between the pin and the conductive layers that are electrically connected at the through hole. The larger the percentage barrel fill, the greater the mechanical stability and electrical connectivity. Further, greater percentage barrel fill results in a larger interface area between the solder and pin. Over time, portions of the solder may crack, and the larger interface area provides more portions that remain in contact which leads to longer usable life of the device.